The present invention relates to a hydraulically operated continuously variable transmission, and more particularly to a hydraulically operated continuously variable transmission including a hydraulic pump coupled to an input shaft and a hydraulic motor coupled to an output shaft, the hydraulic pump and the hydraulic motor being interconnected by a closed hydraulic circuit.
There have been proposed various hydraulically operated continuously variable transmissions for use in automobiles, including a swash-plate axial-plunger hydraulic pump having a pump cylinder coupled to an input shaft and a plurality of pump plungers slidably disposed in the pump cylinder in an annular pattern around an axis of rotation of the pump cylinder, the hydraulic pump having an outlet port, a swash-plate axial-plunger hydraulic motor having a motor cylinder coupled to an output shaft and a plurality of motor plungers slidably disposed in the motor cylinder in an annular pattern around an axis of rotation of the motor cylinder, the hydraulic motor having an inlet port, and a closed hydraulic circuit interconnecting the hydraulic pump and the hydraulic motor. Power is transmitted by the transmission through relative rotation between the motor cylinder and the pump cylinder.
As disclosed in Japanese Patent Publication No. 56-50142, for example, such a continuously variable transmission is controlled such that the transmission ratio will be 1:1 while minimizing or eliminating the angle of inclination of the swash plate of the motor.
By breaking the hydraulic circuit which interconnects the outlet port of the hydraulic pump and the inlet port of the hydraulic motor, the pump plungers can be locked in the pump cylinder to prevent the pump cylinder and the swash plate of the pump from being relatively displaced and to drive the motor cylinder mechcanically through the pump swash plate. With the hydraulic motor and pump thus mechanically locked, the oil pressure discharged by the hydraulic pump is prevented from acting on the motor plungers to reduce the thrust load on the motor swash plate and also to reduce oil leakage from between the motor plungers and the motor cylinder.
Where a shoe is mounted on the distal ends of the motor plungers and a hydraulic pressure is introduced to the sliding surface of the shoe for balancing the hydraulic pressures, the hydraulic pressure on the sliding surface of the shoe can be lowered and any oil leakage can be reduced by breaking the hydraulic circuit.
It is known that it is possible to improve the power transmission efficiency of the transmission and increase the durability of the transmission by breaking the hydraulic circuit between the hydraulic pump and the hydraulic motor.
When the hydraulic circuit is broken by a circuit making/breaking device at the time the transmission ratio is minimal, since the volumetric efficiency becomes higher than before the circuit is broken, the engine rotational speed is lowered as indicated by the following equations: ##EQU1##
Therefore, if the hydraulic circuit is broken when the pressure of operating oil is high, i.e., the engine load is relatively high, the engine rotational speed is abruptly lowered, producing a shock that can be felt by the driver, and the smooth running, as it is felt by the driver, of the vehicle is impaired.